In 14 insulin dependent diabetics past their initial remission period B-cell function was evaluated using a test meal before and after 1 week of strict blood glucose control, and again 3 weeks later when the patients were outpatients on conventional therapy. Eight patients with fasting C-peptide above 0.07 nmol/l improved their B-cell function significantly (p Less Than 0.05) during the period of strict blood glucose control. However, the improvement was of short duration and was absent 3 weeks later in most patients. Six patients with fasting C-peptide below or equal to 0.07 nmol/l had no significant improvement in B-cell function during the period of strict control. The study shows that B-cell function and degree of blood glucose control are related in patients with fasting C-peptide above 0.07 nmol/l, and that B-cell function can change within days.